1. Field
The following description relates to a time and wavelength division multiplexing-passive optical network (TWDM-PON), and more particularly, to wavelength tuning procedures when the TWDM-PON adds an operating channel.
2. Description of the Related Art
A passive optical network (PON) is a subscriber network that connects a central office and a subscriber with a point-to-multipoint topology and is cost effective compared to a structure having a point-to-point topology since required central office systems and optical cables can be reduced.
A time division multiplexing-passive optical network (TDM-PON), for example, Ethernet PON and Gigabit-Capable PON (GPON), uses one wavelength for upstream traffic and another wavelength for downstream traffic to connect a central office to subscribers, and is characterized by its use of, especially, an optical splitter which does not require power to establish a connection between the central office and the subscribers. Thanks to such characteristics, TDM-PON has been distributed worldwide and established successfully. Particularly, GPON networks have been established across the globe, especially in Northern America and Europe. In 2010, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) completed recommendation of G.987 XG-PON standard (10G-GPON). Recently, early commercial products based on the G.987 are being released. Furthermore, the Full Service Access Network (FSAN) Group, which is a standardization group consisting of major communication operators and equipment manufacturers associated with optical subscriber network technology, adopted a time and wavelength division multiplexing passive optical network (TWDM-PON), which is a hybrid type passive optical network that uses time division multiplexing and wavelength division multiplexing at the same time, as major technology of a next-generation passive optical network (NG-PON2). Therefore, the ITU-T is discussing recommendation for G.ngpon2.x standards.
FIG. 1 is a diagram illustrating an optical subscriber network that can accommodate a plurality of different services, for example, TDM-PON (corresponding to GPON OLT and XGPON OLT in FIG. 1), p-to-p (corresponding to OTDR in FIG. 1), RF vide overlay (corresponding to RF VIDEO HE in FIG. 1), and the like. In the system configuration of FIG. 1, NG-PON2 (corresponding to NG-PON2 OLT port-1, . . . , and NG-PON2 OLT port-n in FIG. 1) is a hybrid network that uses TDM and WDM schemes. NG-PON2 with a structure capable of accommodating a plurality of homogeneous or heterogeneous service links using a plurality of optical signals of different wavelengths can advantageously increase the transmission capabilities in proportion to the number of optical wavelength channels, without changing a splitter used in the existing TDM network.
Referring to FIG. 1, a TWDM-PON network represented as NG-PON2 is a hybrid passive optical subscriber network that accommodates a central office system, including n optical line terminals (OLTs) that use different wavelengths. Under the assumption that each central office system accommodates one PON link, one optical distribution network accommodates n homogeneous or heterogeneous networks, and services are distinguished from each other by a wavelength band of a signal used. In this case, TWDM-PON optical network unit (ONU) (NG-PON2 ONU) receives wavelength-multiplexed downstream optical signals from a plurality of TWDM-PON OLTs. To communicate with a particular TWDM-PON OLT, the TWDM-PON ONU should be able to select a wavelength of an upstream signal corresponding to the particular TWDM-PON OLT. Thus, the ONU needs to be equipped with a wavelength-selectable transceiver, that is, a tunable transceiver. The tunable transceiver includes a tunable laser and a tunable receiver.
FIG. 2 is a conceptual diagram of TWDM-PON as a main technology of the next-generation passive optical subscriber network. In FIG. 2, it is assumed that there are n OLTs using different wavelengths, and each OLT accommodates one PON link. A single splitter accommodates n TDW-PON networks, and TDM-PON links are distinguished from each other by a different wavelength used.
In the system of FIG. 2, one or more ONUs that use the same wavelength (λd1 for downstream transmission, λu1 for upstream transmission), for example, ONU A communicates with OLT #1 that uses the same wavelength, and in similar manner, ONU B may be connected to OLT #2. An upstream signal is transmitted to an OLT using a wavelength that matches a downstream signal chosen by the ONU or a wavelength indicated by wavelength allocation information received from the OLT, and upstream signals transmitted from a plurality of ONUs are separated by wavelength by a demultiplexer, and resultant signals are transmitted to corresponding OLTs.
By contrast, since a downstream signal is multiplexed by a wavelength-multiplexer, each ONU receives all downstream wavelengths, and each ONU utilizes only a downstream signal of a particular wavelength chosen from the received all downstream wavelengths. To this end, each ONU needs to select a wavelength of a downstream signal and a tunable receiver has to be synchronized with the downstream signal of the selected wavelength. Efforts to standardize a process of tuning a wavelength in an ONU is now under way in the international standard ITU-T G.989.3, in which an ONU may receive a downstream signal through synchronization of a selected arbitrary downstream signal and may be activated based on the received downstream signal so as to connect and communicate with an OLT.
In TWDM-PON system, it is possible to increase or decrease the number of currently operating channels, that is, the number of pairs of downstream wavelength and upstream wavelength for channel load balancing or efficient system management. For example, the TWDM-PON may use all four channels when there are many service users, but when the number of service users decreases or the needed traffic capacity is reduced, it may be possible to reduce the number of channels in use for efficient system operation.
FIG. 3 is a diagram schematically illustrating a relevant scenario. Referring to FIG. 3, at normal stage, a service is provided through downstream wavelengths λ1, λ2, λ3, . . . , and λn of all channels, while at power saving stage, downstream wavelengths λ2, μ3, . . . , and λn of some channels stop being used, and the service is provided through only the downstream wavelength λ1 of the remaining channel. Therefore, by shutting the power to the optical transceivers of OLTs (corresponding to NG-PON2 OLT Port-2, NG-PON2 OLT2 OLT Port-3, . . . , and NG-PON2 OLT Port-n in FIG. 3) for unused channels, the power consumption by the system can be reduced.
In another example, the TWDM-PON uses only some channels in an initial stage, but if there is an increase in the number of service subscribers or in traffic, channels to be used may be added to guarantee the quality of services. That is, in order to reduce system operation costs, communication service providers that provide Internet services using the TWDM-PON may use only one channel in the initial stage, and may gradually increase the number of operating channels. FIG. 4 is a diagram schematically illustrating a relevant scenario. Referring to FIG. 4, a service is provided through one downstream wavelength channel (λ1) in the initial stage, through two downstream wavelength channels (λ1 and λ2) in the intermediate stage, and through all the downstream wavelength channels (λ1, λ2, λ3, . . . , λn) in the final stage.
However, as can be seen in the initial stage or in the intermediate stage, an ONU that is newly activated in the TWDM-PON that uses only some of the operating channels may identify and store only the downstream wavelength channels that are currently used. Further, the ONU transmits an upstream signal to an OLT using an upstream wavelength that corresponds to the identified downstream wavelength. Then, in the case where there is an increase in the number of subscribers or in traffic, the OLT increases the number of channels used to provide services, and transmits a command or request of wavelength change or wavelength tuning to some or all the ONUs to change previous channels to newly added channels.
FIG. 5 is a diagram partially illustrating an example of wavelength tuning which is described in ITU-T G. 989.3, more specifically G. 989. 3 (Title: Draft new recommendation ITU-T G. 989.3 (for Consent, 4 Apr. 2014)) that is currently in the process of being standardized by ITU-T Study Group 15. The example of wavelength tuning illustrated in FIG. 5 is the case of changing a channel, in which a channel of ONU1 is changed from a first channel (λ1d, u) allocated to an OLT-port 1 to a second channel (λ2d, u) allocated to an OLT-port 2. Referring to FIG. 5, an OLT, more specifically the OLT-port 1, transmits a wavelength-tuning request message, e.g., a Tuning_Control (Request) PLOAM message, to an ONU1 to change a channel to the second channel (λ2d, u) in S10. The second channel (λ2d, u) is a newly added operating channel after the ONU1 has been activated in the TWDM-PON.
Once the ONU1 receives the request message through the downstream wavelength (λ1d) of the first channel (λ1d, u), the ONU1 determines whether a wavelength may be changed, and transmits a response message that includes a determination result to the OLT-port 1 through the upstream wavelength (λ1u) of the first channel (λ1d, u). Generally, in the case where the ONU1 may change a wavelength in response to a wavelength change request, the ONU1 transmits an ACK message, e.g., a Tuning_Response (ACK) PLOAM message, which indicates that a wavelength is tunable, and in the case where the ONU1 may not change a wavelength, the ONU1 transmits an NACK message, e.g., a Tuning_Response (NACK) PLOAM message, which indicates that wavelength is not tunable. However, in the above case, the ONU1 that receives a channel change request has no choice but to transmit an NACK message, e.g., a Tuning_Response (NACK) PLOAM message, to the OLT-port 1 in S11. The reason is that the ONU1 has identified and stored information on the downstream wavelengths of channels used when the ONU1 has been activated to establish a link through the first channel (λ1d, u), but has no information on a downstream wavelength of a newly added channel, i.e., the second channel (λ2d, u). As a result, in the case where the number of channels used in the system is increased, an ONU may not change a previously used channel to a newly added channel by using the general wavelength tuning process.